Sensing temperature of a light emitting diode

ABSTRACT

A system ( 100 ) for sensing a temperature of a light emitting diode (LED). The system may comprise an LED having a spectral output centered at a first wavelength, a first filter ( 104 ) that transitions from attenuation to transmission at about the first wavelength, and a second filter ( 106 ) that transitions from transmission to attenuation at about the first wavelength. The system may also comprise a first sensor ( 108 ) positioned to sense a first intensity of the LED through the first filter and a second sensor ( 110 ) positioned to sense a second intensity of the LED through the second filter. It will be appreciated that a single sensor may be substituted instead of the first and second sensors, provided that the single sensor is capable of selectively viewing the LED through the first and the second filters. The system may also comprise a computer ( 112 ) configured to derive a temperature of the LED considering the first intensity and the second intensity.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to and is a U.S. national phaseapplication corresponding to International Application No.PCT/US2006/031890 entitled “Sensing Temperature of a Light EmittingDiode”, which designated the United States and was filed on Aug. 15,2006, published in English. The entire content of the foregoing patentapplication is incorporated herein by reference.

BACKGROUND

For many years, Light Emitting Diodes (LED's) have provided anattractive alternative to traditional incandescent and fluorescentsources because of their small size and energy efficiency. In opticalmeasurement systems, and other applications that benefit from consistentand/or predictable light sources, however, LED's have been slower tocatch on. This is because the spectral output of an LED, both in termsof intensity and wavelength, varies greatly with temperature. Attemptshave been made to control this variation, for example, by varying thecurrent and/or voltage of LED's or even by heating the LED's prior touse. These methods, however, add additional complexity and expense and,for many applications, still fail to deliver an acceptable level ofconsistency.

SUMMARY

In one general aspect, the invention is directed to a system for sensinga temperature of a light emitting diode (LED). The system may comprisean LED having a spectral output centered at a first wavelength, a firstfilter that transitions from attenuation to transmission at about thefirst wavelength, and a second filter that transitions from transmissionto attenuation at about the first wavelength. The system may alsocomprise a first sensor positioned to sense a first intensity of the LEDthrough the first filter and a second sensor positioned to sense asecond intensity of the LED through the second filter. It will beappreciated that a single sensor may be substituted instead of the firstand second sensors, provided that the single sensor is capable ofselectively viewing the LED through the first and the second filters.The system may also comprise a computer configured to derive atemperature of the LED considering the first intensity and the secondintensity.

In another general aspect, the invention is directed to methods ofdetermining a temperature of an LED. The methods may comprise the stepsof sensing a first intensity of the LED through a first filter andsensing a second intensity of the LED through a second filter. The firstfilter may transition from attenuation to transmission at about a peakwavelength of the LED, and the second filter may transition fromtransmission to attenuation at about the peak wavelength of the LED. Themethods may also comprise the step of calculating a temperature of theLED considering the first intensity and the second intensity.

In yet another general aspect, the invention is directed to methods ofcalibrating a system for determining a temperature of a light emittingdiode (LED). The methods may comprise the step of activating the LED ata first known temperature. The methods may also comprise the steps ofsensing a first intensity of the LED through a first filter, and sensinga second intensity of the LED through a second filter. The first filtermay transition from attenuation to transmission at about a peakwavelength of the LED, and the second filter may transition fromtransmission to attenuation at about the peak wavelength of the LED. Themethods may also comprise the step of relating the first intensity andthe second intensity to the first temperature.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the present invention are described herein, by way ofexample, in conjunction with the following figures, wherein:

FIG. 1 shows a diagram of a system for measuring the temperature of alight emitting diode (LED) according to various embodiments;

FIG. 2A shows a chart of various response curves of an LED at differenttemperatures;

FIG. 2 shows a chart of the response curves of an LED and a pair ofsensors according to various embodiments;

FIG. 3 shows a flowchart of a process flow for calibrating a system formeasuring the temperature of an LED according to various embodiments;and

FIG. 4 shows a flowchart of a process flow for measuring the temperatureof an LED according to various embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are directed to systems and methodsfor measuring the temperature of a light emitting diode (LED). LEDtemperatures measured according to various embodiments may be used forany suitable purpose. For example, the spectral output of an LED over arange of temperatures may be generalized based on models of theunderlying physics and/or experimental characterizations. Accordingly,the measured temperature of an LED may be used to derive an indicationof the LED's spectral output. Also, for example, the temperature of anLED taken near the time when the LED is first illuminated may indicatethe ambient air temperature surrounding the LED.

FIG. 1 shows a block diagram of a system 100 for measuring thetemperature of a light emitting diode (LED) 102. In addition to the LED102, the system 100 may include one or more sensors 108, 110, with apair of optical filters 104, 106 positioned between the LED 102 and thesensors 108, 106. The sensors 108, 110 may sense the output of the LED102 through the respective filters 104, 106. Two sensors 108, 110 areshown, however, it will be appreciated that one of the sensors 108, 110may be omitted, for example, if the remaining sensor is able toselectively view the output of the LED 102 through both of the filters104, 106. The system 100 may also comprise a computer 112 or othersuitable processing device to store and analyze signals from the sensors108, 110.

It will be appreciated that the spectral output of the LED 102 may varywith temperature in a predictable way. For example, the materials usedto produce LED 102, such as GaAs, GaN, etc., have inherent dispersiveproperties (e.g., dielectric constant, complex refractive index, etc.)that vary with both wavelength and temperature (dn/dT and dn/dλ). As aresult, the LED 102 may exhibit behaviors that vary proportionately toλ²/Δλ, as the temperature, forward current and/or forward voltagechange. Also, as temperature increases, the physical dimensions of LED's105 dies may change. This, together with the changes in dispersion, mayresult in a net shift of the peak wavelength, λ, output, a decrease inlight output, and a change in the bandwidth, Δλ, as the temperature ofthe LED's 105 change.

For example, FIG. 2A shows a general shape of a model 250 of thespectral output of an LED, according to various embodiments. Threecurves 252, 254 and 256 are shown representing the output of the LED atthree different temperatures. The curve 252 shows the spectral output ofthe LED at a first temperature. The curve 254 shows the spectral outputof the LED at a second temperature higher than the first. Finally, thecurve 256 shows the LED at a third temperature lower than the firsttemperature. It can be seen that, generally, as temperature increases,the LED's spectral output may generally increase in bandwidth anddecrease in intensity.

FIG. 2 shows a diagram 200 of the spectral responses of the LED 102 andfilters 104, 106 in conjunction with the corresponding sensor or sensors108, 110. Curve 202 represents the spectral output of the LED 102, whilecurves 204 and 206 represent the spectral responses of the filters 104and 106, respectively. The filters 104, 106, as shown by filter curves204 and 206, may be chosen to have adjacent or roughly overlappingattenuation bands at about the peak wavelength of the LED 102, shown byLED curve 202. In various embodiments, the LED 102 may be chosen with anominal peak wavelength of 590 nm, while the filter 104 (curve 204) maybe a green band-pass filter and the filter 106 (curve 206) may be a redband-pass filter. It will be appreciated, however, that the system 100may include any suitable LED and filter combination. For example, a 505nm LED and/or a 525 nm LED could be used in conjunction with blue andgreen filters.

As described above, it will be appreciated that as the temperature ofthe LED 102 changes, the position and/or shape of the curve 202 willalso change in a predictable way. For example, as the temperature of theLED 102 increases, the LED spectral output 202 may be shifted to alonger wavelength (to the right in the diagram 200). When this occurs,more of the LED's total output may be attenuated by filter 104 (curve204), and more of the LED's total output may be passed by the filter 106(curve 206). As the temperature of the LED 102 decreases, the oppositemay occur. Accordingly, the peak wavelength of the LED 102, andtherefore its temperature, may be sensed by comparing the intensity ofthe LED 102 as viewed through filter 104 to the intensity of the LED 102as viewed through filter 106.

FIG. 3 shows a process flow 300, according to various embodiments,illustrating a method for calibrating the temperature measuring system100. It will be appreciated that the steps of the process flow 300 maybe performed in any suitable order, and that some or all of the stepsmay be performed simultaneously. At step 302, the LED 102 may beactivated at a first known temperature. The intensity of the LED 102through the filter 104 may be measured at step 304. The intensity of theLED 102 through the filter 106 may be measured at step 306. At step 308,the computer 112 may create or supplement a model relating thetemperature of the LED 102, the intensity of the LED 102 through thefilter 104 and the intensity of the LED 102 through the filter 106(e.g., the model shown above at FIG. 2A). In various embodiments,creating or supplementing the model may involve calculating one or morecoefficients matching the observed intensities to the model. In variousembodiments, the computer 112 may also explicitly solve for the peakwavelength of the LED 102.

At decision step 310, the computer may determine whether additionalmeasurements will be taken to further supplement the model. If anadditional measurement is desired, the temperature of the LED 102 may bechanged at step 312. For example, the temperature of the LED 102 may bevaried by allowing it to be activated for a given period of time,activating additional LED's near the LED 102, etc. The process may thencontinue with step 304 as described above. It will be appreciated thatone measurement may be sufficient to develop the model, however,additional measurements may improve the accuracy of the model. Also,taking measurements over a broad range of temperatures or otheroperating conditions may allow the model to compensate fornonlinearities in LED heating behavior, the effects of additional LED's(not shown) near the LED 102, etc.

FIG. 4 shows a process flow 400, according to various embodiments, formeasuring the temperature of the LED 102 using the system 100. At step402, the LED 102 may be activated. The intensity of the LED 102 throughthe filter 104 may be measured at step 404, and the intensity of the LED102 through filter 106 may be measured at step 406. It will beappreciated that the respective intensities of the LED 102 throughfilters 104 and 106 may be measured near the time that the LED 102 isactivated, or at any time thereafter. At step 408, the first and secondintensities of the LED 102 may be used to calculate a temperature of theLED 102, for example, according to a model generated as described above.The temperature of the LED 102 may then be used in any suitable way, forexample, as described above. In various embodiments, the LED 102 may bepart of an array of LED's positioned in close proximity to one another.It will be appreciated that, in this case, other LED's included in thearray may be assumed to have the same temperature as the LED 102. Thisassumption is likely to be more accurate where all of the LED's in thearray are activated for similar amounts of time under similarconditions.

It is to be understood that the figures and descriptions of the presentinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the present invention, while eliminating,for purposes of clarity, other elements, such as, for example, detailsof various physical models of LED's, etc. Those of ordinary skill in theart will recognize that these and other elements may be desirable.However, because such elements are well known in the art and becausethey do not facilitate a better understanding of the present invention,a discussion of such elements is not provided herein.

As used herein, a “computer” or “computer system” may be, for exampleand without limitation, either alone or in combination, a personalcomputer (PC), server-based computer, main frame, server, microcomputer,minicomputer, laptop, personal data assistant (PDA), cellular phone,pager, processor, including wireless and/or wireline varieties thereof,and/or any other computerized device capable of configuration forprocessing data for standalone application and/or over a networkedmedium or media. Computers and computer systems disclosed herein mayinclude operatively associated memory for storing certain softwareapplications used in obtaining, processing, storing and/or communicatingdata. It can be appreciated that such memory can be internal, external,remote or local with respect to its operatively associated computer orcomputer system. Memory may also include any means for storing softwareor other instructions including, for example and without limitation, ahard disk, an optical disk, floppy disk, ROM (read only memory), RAM(random access memory), PROM (programmable ROM), EEPROM (extendederasable PROM), and/or other like computer-readable media.

The computer 112 may operate according to software code to be executedby a processor(s) of the computer 112 or any other computer system usingany type of suitable computer instruction type. The software code may bestored as a series of instructions or commands on a computer readablemedium. The term “computer-readable medium” as used herein may include,for example, magnetic and optical memory devices such as diskettes,compact discs of both read-only and writeable varieties, optical diskdrives, and hard disk drives. A computer-readable medium may alsoinclude memory storage that can be physical, virtual, permanent,temporary, semi-permanent and/or semi-temporary. A computer-readablemedium may further include one or more data signals transmitted on oneor more carrier waves.

While several embodiments of the invention have been described, itshould be apparent that various modifications, alterations andadaptations to those embodiments may occur to persons skilled in the artwith the attainment of some or all of the advantages of the presentinvention. It is therefore intended to cover all such modifications,alterations and adaptations without departing from the scope and spiritof the present invention as defined by the appended claims.

1. A system for sensing a temperature of a light emitting diode (LED),the system comprising: the LED having a spectral output centered at afirst wavelength; a first filter that transitions from attenuation totransmission at substantially the first wavelength; a second filter thattransitions from transmission to attenuation at substantially the firstwavelength; at least one sensor positioned to sense a first intensity ofthe LED through the first filter and a second intensity of the LEDthrough the second filter; and a computer configured to derive atemperature of the LED considering the first intensity and the secondintensity.
 2. The system of claim 1, wherein the at least one sensorcomprises a first sensor and a second sensor, and wherein the firstfilter is positioned between the LED and the first sensor and the secondfilter is positioned between the LED and the second sensor.
 3. Thesystem of claim 1, wherein the at least one sensor comprises a firstsensor configured to selectively view the LED through the first filterand the second filter.
 4. The system of claim 1, wherein the firstwavelength is approximately 590 nm.
 5. The system of claim 1, whereinthe first filter is a green band-pass filter and the second filter is ared band-pass filter.
 6. A method of determining a temperature of alight emitting diode (LED), the method comprising: sensing a firstintensity of the LED through a first filter, wherein the first filtertransitions from attenuation to transmission at substantially a peakwavelength of the LED; sensing a second intensity of the LED through asecond filter, wherein the second filter transitions from transmissionto attenuation at substantially the peak wavelength of the LED; andcalculating a temperature of the LED considering the first intensity andthe second intensity.
 7. The method of claim 6, further comprisingcalculating a peak wavelength of the LED based on the first intensityand the second intensity.
 8. The method of claim 6, further comprisingcalculating a spectral output of the LED considering the calculatedtemperature.
 9. The method of claim 6, further comprising calculating aspectral output of a second LED positioned adjacent the first LEDconsidering the calculated temperature.
 10. The method of claim 6,further comprising activating the LED.
 11. A method of calibrating asystem for determining a temperature of a light emitting diode (LED),the method comprising: activating the LED at a first known temperature;sensing a first intensity of the LED through a first filter, wherein thefirst filter transitions from attenuation to transmission atsubstantially a peak wavelength of the LED; sensing a second intensityof the LED through a second filter, wherein the second filtertransitions from transmission to attenuation at substantially the peakwavelength of the LED; and relating the first intensity and the secondintensity to the first temperature.
 12. The method of claim 11, furthercomprising: changing the temperature of the LED to a second knowntemperature; sensing a third intensity of the LED through the firstfilter; sensing a fourth intensity of the LED through the second filter;and relating the third intensity and the fourth intensity to the secondtemperature.
 13. The method of claim 11, further comprising: activatingat least one LED positioned near the LED; sensing a third intensity ofthe LED through the first filter; sensing a fourth intensity of the LEDthrough the second filter; and relating the third intensity and thefourth intensity to the second temperature.